Polymer particles are used in a polymer matrix to manage light diffusion. For example, U.S. Pat. No. 7,547,736 describes the use of particles having an average particle size of 15 to 70 microns to provide a frosted look and textured surface, and U.S. Pat. No. 8,163,827 describes a high light transmission diffusion screen having inorganic pigments and refractive index-matched particles.
Point light sources create a visible shape of the light source, and there is often a desire to hide the light source shape, creating a more diffuse lighting. As used herein, “point light source” means any shaped source of electromagnetic radiation in the 3,600-7,700 Angstrom range. This includes, but is not limited to, incandescent, fluorescent, neon, argon and LED light sources.
Light emitting diodes (LEDs) are being increasingly favored as a light source, since they use far less electricity and produce less heat than standard incandescent or fluorescent light bulbs. LEDs provide a very bright, point light source, yet the output (7000° K) often appears harsh and causes an uncomfortable glare. This is a challenge for lighting designers, as many applications demand an illumination and low glare. Light devices that integrate one or more LEDs include, for example, luminaires for commercial or residential lighting, motor vehicle illumination devices (at the front or rear), indicating panels, luminous displays, spotlights, street lighting, box-letters, etc.
These luminous devices typically consist of a light source and a cover (also called a lens or a diffuser) made of a plastic whose function is to mask and protect the light source, while still ensuring good transmission of the light emitted by the light source. The plastic may be colored or may have decorative elements or patterns. The cover also has the function of scattering the emitted light so that the illumination is softened and not dazzling. The scattering of the light emitted by the light source can be achieved by dispersing scattering particles of organic or mineral nature in the plastic.
Replacing a conventional light source with an LED results in a modification of the illumination. This is because an LED, especially an LED having a high luminous flux, exhibits directional illumination, whereas, for example the illumination of a neon tube is from 0 to 360°. In addition, the emission spectrum of an LED is completely different from that of a conventional light source.
WO 2006/100126 describes a thermoplastic cover with dispersed beads for use with LEDs to form luminous devices. 3-30% of scattering particles are dispersed in a transparent plastic. The particles can be inorganic or organic and have mean diameters of from 0.5 to 100 microns. There is no description of combinations of particle size and loading, and no teaching of hiding power.
The addition of scattering particles helps to soften the effect of the LED light source, but the scattering also reduces light transmission. Some LED lighting cover manufacturers add pigments, such as BaSO4, SiO2, CaCO3, AL2O3, TiO2 and ZnO (U.S. Pat. No. 4,418,986), to the cover to increase the hiding power, though this can dramatically decrease the light transmission. BaSO4 and refractive-index-matched beads are used in a light diffusion (TV) screens in U.S. Pat. No. 8,163,827. Ground-up cell-cast sheet forms irregular particles with a wide particle size distribution, and unsatisfactory light transmission properties for LED diffusion sheets.
WO 14/055330 describes an attempt to balance the light transmission and hiding power for an LED diffuser, using a combination of different sized plastic beads.
It has now been found that good diffusion sheet for use with LED point light sources can be achieved by forming a selected blend of inorganic and organic diffusion particles in a plastic matrix. The addition of optical brighteners can further improve the color properties of a diffusion article formed from the blend.